The heating field of the window is frequently used in vehicle antennas as the antenna structure. The heating field is provided for both FM and TV reception in U.S. Pat. No. 6,498,588 and in PCT Published International Application No. 99/66587. A conductor loop, which is not connected to the heating field, is additionally provided for LMS (long, medium, short wave) reception on the upper edge of the window.
A significant disadvantage of such a system is the necessity of a surface, e.g., in the upper area of the window, which, due to non-existing heating conductors, cannot be heated and therefore cannot be defrosted. The available heatable area is unacceptably small, particularly in passenger cars having small windows.
The heating conductors run essentially horizontally and essentially parallel to the metallic boundaries of the window. Interference in the vehicle electrical system, transferred by the heating current to the heating conductors acting as the antenna, must, as is known, be suppressed via modules having a high-resistance behavior at high frequencies when the antenna connection point is galvanically linked to the heating field. For FM reception, these modules are one core double chokes, for example, which are integrated into the heating current-supplying conductor segments and, as a rule, are situated at the heating current terminals of the heating field.
For LMS reception, this is a current-compensated toroidal core choke (AM blocking circuit) which is also situated in the heating current supply lead. This AM blocking circuit is a very cost-intensive module whose dead weight of approximately 200 g results in high mechanical stresses on the printed circuit board as well as the union points, and is thus to be valued extremely critical with respect to quality assurance. The vibrations, occurring during normal driving conditions, result in the fact that soldered joints are extremely stressed. In compact vehicles, this AM blocking circuit is frequently mounted in the tailgate, so that accelerations of approximately 50 g may occur when the tailgate is slammed shut and the module breaks off the union points.
In other vehicle antenna reception systems, the reception of LMS and diverse FM signals is implemented via conductor structures in one or multiple window panes which are for the most part situated in the immediate proximity of, but spatially separated from, one another. A significant disadvantage in such a system is the necessity of at least two, for the most part fixed, window panes which results in increased expenses for the manufacture of the panes, for the electronic design of connected, for the most part active circuit components, and for the assembly of appropriate circuit carriers.
Antenna systems are also known which form the antennas for LMS and FM reception from the galvanically contacted heating field. Here also, filter elements, which decouple the vehicle electrical system, are necessary for the antenna connection point and the heating conductor (European Published Patent Application No. 0 269 723 and European Published Patent Application No. 0 382 895).